1. Field of the Invention
The present invention relates generally to the deposition of image producing screens for cathode ray tubes (CRTs). Most particularly, the present invention relates to screen application, also referred to as screening, by photoexposure of the screen elements using a standardized photographic stencil, or artwork plate, placed in proximity to a screening surface of a CRT front panel, or faceplate, during the photoexposure.
2. Discussion of the Related Art
CRTs or tubes are commonly screened today through a mated-mask process wherein a shadow mask is placed in its actual operational position at a distance "Q" from the front panel and used as the photographic "stencil" for the photoexposure deposition of the screen elements on one CRT panel. This technique, wherein the stencil is placed at, or substantially at, the "Q" distance is denominated as "projection" printing. As used herein the term "photostencil" or "stencil" will refer to an impervious material having a transmissive aperture pattern for the purpose of allowing radiant emissions to pass therethrough onto photosensitive layers of CRT screen elements in order to create a desired matrix, or pattern, of screen elements. The term "stencil" should not be taken to mean an apertured, or perforate, negative pattern designed to reproduce screen elements of the exact size and placement as the apertures in one to one correspondence onto the screen surface substrate of the CRT.
In general, the screen is formed by serial deposition and exposure of photosensitive slurries of the grille and phosphor materials deposited on a screening surface of the front panel of the CRT. The projection photoexposure process uses light directed through a lens to simulate the path of electron beams in the assembled tube. Thus, the exposure light is used to form the phosphor elements, and the electron beams impinge upon these phosphor elements to produce an image. The panel and mask are, of necessity, mated, or dedicated to each other, throughout the CRT assembly process so that when the CRT is assembled, the mask used to form the screen through photoexposure is also the mask used to control placement of the phosphor-exciting electron beams in tube operation. Thus, screen placement of the phosphors by photoexposure through the mask, corresponds, or registers, with the electron beam placement on the phosphors, which is also controlled through the same shadow mask. Hence, no misregistration between mask apertures and phosphor deposits occurs during tube operation and a suitable image is produced by the screen.
Projection photoexposure screening utilizing a mated mask and screen presents logistical problems, with associated manufacturing expense, in keeping the mask and screen together at all relevant times.
Also, due to diffusion and diffraction effects of projection photoexposure screening, the processing and photochemistry parameters must be tightly controlled in order to achieve acceptable yields of screens. Further, using the common screen element photochemistry and projection exposure techniques there is likely an absolute limit on the number of discrete individual phosphor dots which can be placed on the screen before morphological distortion of the discrete screen elements, e.g. phosphor dots, will cause the elements to run together. This limitation is particularly significant because higher resolution tubes towards which the industry is moving, must have a greater number of smaller phosphor dots or lines on the same screening surface area. For example, a current fourteen inch diagonal measure high resolution monitor has approximately 2.4 million phosphor dots, whereas proposed high resolution tubes may require about 5.5 million dots in the same screen area.
As shown in U.S. Pat. No. 4,248,947 to Oikawa, contact photoexposure screening uses a standardized, or master, exposure pattern for all tubes in the screening process. A plate carrying the master pattern is placed directly in contact with the screen elements, i.e. the grille and phosphor components, and allows for exposure of the screen element composition layers by use of floodlights. Contact exposure thus eliminates the need for a corrective lens and other such optical controls which are used to make the exposure light conform to electron beam behavior during exposure. This is because the light, or other radiant emissions, used to develop the screen can only land in one place on the screen, i.e., directly beneath the photostencil apertures. However, because of this, contact exposure requires very close attention to the physical dimensions of the tube during manufacture because of the difficulty in compensating for changing mask and screen geometry, and consequent electron beam misregistration, due to dimensional irregularities between tubes. Any change in mask or screen geometry when using a standardized stencil may result in electron beam misregistration on the screen, causing color impurities in the image.
Further, in contact photoexposure the standardized photostencil may remove portions of the screen element slurry which adhere to it, necessitating frequent and thorough cleaning of the stencil in order to avoid deposition of the adhered slurry on subsequent screens or subsequent mispositioning of the stencil due to the adhered slurry acting as a spacer on the stencil.
An early system which places a standard photographic stencil in near contact, or proximity, to the screening surface, which will be understood to be coated with a layer of photosensitive screen element composition during exposure, has been described in U.S. Pat. No. 3,973,964 issued to Howard A. Lange. The above-cited parent disclosure, U.S. Pat. No. 4,902,257 issued to Robert Adler et al., also of common ownership herewith, suggests a proximity printing apparatus offering the advantage of screen element placement compensation according to varying tube dimensions during exposure, while reducing undesirable diffusion and diffraction problems. Both of these systems seek to achieve a CRT using interchangeable screens and masks wherein any front panel may be mated with any mask to create an optimally functional CRT.
As noted in the above-cited parent disclosure proximity printing may be utilized without resort to "rail" height references when the Q height is held to a very close tolerance. As also noted, this Q height tolerance control may be alleviated significantly where the proximity print process utilizes the rail height for setting the proximity distance hereinafter sometimes referred to as Q', between the photostencil and the screening surface. As further noted therein, offset printing of a standardized screen may require special control of the accuracy of the Q height. Thus, a variety of apparatus and methods may be utilized depending upon the standardized screening method selected.
By combining a proximity printing system with a simplified tube geometry such as exists in the flat tension mask technology of the present owner's unique CRT's (see FIG. 1), high resolution screens may be made with well understood processes and chemistries to improve yields while effecting economies in time and materials previously not possible. References illustrating aspects of tube manufacture which also may be of interest herein include: Oikawa, U.S. Pat. No. 4,248,947; Fiore, U.S. Pat. No. 3,676,914; Fischer-Colbrie, U. S. Pat. No. 2,842,696; and Grimm et al., U.S. Pat. No. 2,733,366.